Re: Power Amp Repair


"Haaky" <mmartin10@xxxxxxxxx> wrote in message
news:1170788997.011896.9860@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Here we go again:)

I just checked the schematics and i don't see any a1492 or c3856
listed on there at all



It would seem that your transistors have been subbed when it was repaired.
This is not uncommon, and I have on occasion done it myself, although where
possible, I prefer to use original types, even down to actually obtaining
them from the equipment manufacturer, rather than a third party general
spares supplier.

Theoretically, the transistors should indeed match on both channels, as your
schematic suggests. Sometimes however, particular types are either difficult
to obtain, or prohibitively expensive. Most repair people will have their
own favourite substitute types to use in cases like this. Usually, these
substitute parts are chosen to be better rated than the types that they are
replacing, so don't cause a problem. In general, the output transistors are
just meaty current carriers, and for the most part, due to the large amounts
of feedback being employed, and automatic bias stabilisation, their exact
characteristics are not that important. Occasionally though, an output
circuit design is critical of such parameters as transistor gain, and
substitute devices just will not work well in it. This usually manifests
itself, however, as symptoms like long-term overheating, and a failure of
the bias circuit to operate correctly. Also, some output stages use
darlington transistors, which of course cannot be substituted with 'normal'
types, and in fact I would not recommend that they be replaced with anything
other than originals.

Without going and looking up the differences between what was originally
fitted, and what is now, I wouldn't like to say for sure whether they are
valid substitute types, but I'm willing to bet that they will probably be
OK, and are not the cause of the problem, as you said previously that when
it is running, it does not overheat, and sounds fine.

Fuse blowing at switch on is not something that you would normally consider
to be output stage related, unless it is a 'hard' fault such as a short
circuit device, in which case, the amp would never be able to work. I still
think that all you need, is to get the correct fuse type in there, and all
will be well. If the type that is in there is indeed a fast blow, as
Graham's research would suggest, then it is not a valid type for that
position, and will blow in just the way that you are seeing. Also, as Graham
says, slo-blo and " T " rated are not quite the same on paper, but to all
intents and purposes for this discussion, they can both be treated as
'equivalent' anti-surge types. Get the right fuse in there, and then come
back to us in a couple of weeks and tell us that it's OK now.

Arfa


.

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